1. Field of the Invention
The present invention generally relates to a display device and, more particularly, to a display having a capability of automatically controlling the luminance at the screen in response to a change in intensity of ambient light falling on the screen so that displayed information such as characters, images or a combination thereof may be clearly viewed regardless of the ambient brightness or illuminance.
2. Description of the Prior Art
A display device used as an output terminal device of a personal computer, a word-processor or an engineering work station can be found not only in houses, but also in offices. This type of display device makes use of a display unit such as, for example, a cathode ray tube (CRT), a liquid crystal display (LCD) panel, a plasma display panel (PDP) or an electro-luminescent (EL) display panel. The display unit includes a multiplicity of pixels arranged in a matrix pattern to provide a display screen.
Most of the currently available display devices is provided with either an adjustment switch having a plurality of switch positions for intermittently changing the luminance of the display screen or an adjustment knob for progressively changing the luminance of the display screen, so that the operator or user can adjust the luminance of the display screen to render it consistent with the ambient brightness, that is, the intensity of ambient light falling on the display screen.
In either case, in an environment where the ambient light frequently changes, the adjustment of the luminance of the display screen is indeed cumbersome and is often sunk into oblivion. Also, depending on the position of the operator or user, the luminance adjustment may often be time-consuming and good for nothing.
To alleviate those inconveniences, publications such as, for example, the Japanese Laid-open Patent Publication No. 4-20924 and the Japanese Laid-open Utility Model Publication No. 4-75321 disclose the use of an ambient light sensor installed at the front of the display screen to detect the intensity of ambient light falling on the display screen so that an output from the ambient light sensor can be utilized to adjust the luminance of the display screen.
FIG. 6 illustrates, in a block diagram, the principle of the prior art display device disclosed in the first mentioned Japanese publication. Referring to FIG. 6, the prior art display device comprises a liquid crystal panel 1 and a back-lighting panel 3 electrically coupled with a back-light source 4 and positioned parallel to and on one side of the liquid crystal panel 1 opposite to the position of an operator or user (not show, but occupying a left-hand side of the drawing). This prior art display device also comprises an ambient light sensor 2 including an illuminance meter and positioned on one side of the liquid crystal panel 1 opposite to the position of the operator or user for detecting the intensity of ambient light having passed through the liquid crystal panel 1, and a processor 5 operable in response to an output from the ambient light sensor 2 to control the back-light source 4.
According to the prior art display device shown in FIG. 6, the ambient light sensor 2 provides an output S indicative of the intensity of ambient light falling on the liquid crystal panel 1 to the processor 5 which in turn controls the voltage to be applied to the back-light source 4 to adjust the luminance of the back-light source 4 to a value appropriate to the ambient brightness.
The automatic luminance adjustment such as practiced in the prior art display devices is so designed as to minimize a limited amount of electric power and to increase the lifetime of the back-light source. By way of example, in the case of the prior art display device shown in and described with reference to FIG. 6, by lowering the luminance of the back-light source if the ambient illuminance is sufficiently high, the lifetime of the back-light source can be increased.
In any event, the prior art automatic luminance adjustment is such that, when the ambient brightness around the display device is relatively high or low, the luminance of the back-light source is reduced or increased, respectively. This technique is not necessarily consistent with characteristics of human vision.
More specifically, although both an increase in the lifetime of the back-light source and an energy-saving are successfully accomplished, the prior art display device still has a problem in that for a particular operator or user the luminance of the display screen may become either too low to discern the displayed information such as characters, images or a combination thereof, or too high so as to cause glare. This problem often leads to a reduction in work efficiency, asthenopia and/or a sensation of mental fatigue.